One major problem in battery-supported power supply systems is the assessment of the state of charge, of the aging and of the life expectancy of the energy store, irrespective of whether an energy store is used as an emergency battery, as a back-up battery or as a cyclically operated main battery. The need for a valid statement about the storage state of a battery must be regarded as particularly strong when the battery-supported power supplies are used for feeding systems with enhanced safety, security or availability requirements. It is particularly important to determine the battery state in the case of remote, automatically operated island systems, which generally operate without supervision and are not subject to any maintenance for lengthy time periods (weeks, months), since the effort to replace the batteries, for example in the case of maritime navigational aids, or in areas where access is difficult or there is little population, may in some cases be very high.
It is known for batteries to be continuously monitored and for the periodicities for preventative replacement procedures as specified in the operating instructions for such systems to be complied with. The batteries in the system are removed and replaced by new ones irrespective of the actual aging state of the batteries. Ideally, the old batteries are processed, with valuable substances being recycled. This procedure requires a high degree of maintenance effort. Furthermore, the increasingly economical use of resources often prevents batteries from being replaced purely as a preventative measure.
A method for calculation of the capacity balance between assumed ampere hours and emitted ampere hours is widely used. If the initial capacity is known, the no-load current drawn is determined, and the no-load voltage with respect to the capacity is assessed, it is thus possible to assess the state of charge of the battery with sufficient accuracy, particularly when temperature influences are compensated for by means of correction factors. The advantage of this method is the simple evaluation, independently of the chemical processes within the battery. Since the voltage that is produced is always at the same level as the final charge voltage in mains-powered power supply systems, this calculation is always accurate, because the battery is always fully charged when in the balanced state. However, the major disadvantage of this method becomes evident in the case of autonomous systems operated in the island mode. Even if the current detection measurement error is very small, this is integrated over time to produce a very major discrepancy from the actual values, so that the energy balance becomes highly erroneous and unusable for specific state analyses. This problem is worse, in particular in power supplies which supply loads on an event-controlled basis, that is to say in systems in which only a small amount of power is intended to be provided over a specific time period for monitoring purposes, but greater amounts of power are provided for operation of loads when an event is triggered. Furthermore, in the case of autonomous systems, it must always be assumed that the energy balance is not equalized over lengthy time periods. This problem is known, so that the measurement system and the energy store are generally recalibrated at specific times. The store is returned to the fully charged state by means of an external energy supply. However, the monitoring of the measurement system and the independence from the mains system for specific charging of the batteries involve a high degree of complexity.
Another known method relates to the measurement of the voltage at the battery terminals. This makes use of the knowledge that the voltage on a battery represents a measure of the state of charge once the chemical processes associated with charging and discharging have decayed. However, this method is dependent on the battery being removed from the system and being left in a rest state for several hours in order to achieve sufficiently accurate measurements.
Fuzzy algorithms use simulation methods to calculate the instantaneous state of charge based on charging and discharge characteristics from the battery manufacturers and measurement of the current and voltage on the battery. These methods are relatively accurate provided that the battery is operated cyclically, but are subject to errors for battery states of charge above 80%, for small charging and discharge currents and when large temperature differences occur during operation. The system calibration that is required is carried out in the same way as for systems using a current measurement.